A vehicle headlamp or foglamp has been proposed that uses a LED as a light source. For example, a LED-based lamp apparatus disclosed in U.S. Pat. No. 6,870,328B2 corresponding to JP-2004-51014A includes a LED unit constructed with multiple LEDs connected in series. A battery voltage is stepped-up by a booster circuit and then applied to the LED unit. The booster circuit is a chopper circuit constructed with a transistor and a coil. In the booster circuit, a switching action of the transistor is controlled to drive the LED unit by a constant current.
Some of the LEDs of the LED unit may be broken and short-circuited due to, for example, long-term use. In this case, the constant current flowing through the LED unit increases instantaneously and an overcurrent condition occurs. In the LED-based lamp apparatus, the switching action of the transistor of the booster circuit is changed to correct the overcurrent condition.
However, there is a time lag between when the switching action of the transistor is changed and when the current starts to decrease. As a result, the overcurrent condition lasts for a relatively long time and a secondary failure is caused to normal LEDs of the LED unit. Since the LED-based lamp apparatus is used under severe operating conditions such as temperature, and the amount of the current, the secondary failure is likely to be caused due to the overcurrent condition. Therefore, it is preferable to reduce the current as soon as possible after the overcurrent condition occurs.